1. Field of Industrial Use
The present invention relates to a distortion compensating circuit of a high-frequency power amplifier of the kind used in transmitter apparatus and the like.
2. Description of the Prior Art
Because the input/output characteristic of a power amplifier generally is a non-linear characteristic such as that shown in FIG. 1 by a solid line, this characteristic produces non-linear distortion (waveform distortion) dependent on the signal amplitude. In particular, in devices such as portable telephones, because the power source is a battery and low voltage operation is required, when driven at low voltages the non-linear characteristic becomes marked, and non-linear distortion readily occurs.
As methods for compensating this non-linear distortion, the negative feedback method, the predistorter method, and the feed-forward method are being studied.
FIG. 2 shows the distortion compensating circuit of the conventional negative feedback method. In this method, the input signal is supplied to the power amplifier 1 and to an envelope detecting circuit 2, and a detection signal showing the envelope waveform of the input signal is derived. And, after the output signal of the amplifier 1 has been brought to the same level as the input signal by an attenuator circuit 3, it is supplied to an envelope detecting circuit 4, and a detection signal showing the envelope waveform of the output signal is derived. Then, the detection signals of the detecting circuits 2 and 4 are supplied to a subtracting circuit 5 and the difference component between these signals, i.e. the non-linear distortion component, is detected. For instance, the power source voltage of the amplifier 1 is controlled based upon this distortion component, and the amplifier gain is thereby controlled. As a result, an output signal which has been power-amplified and compensated for non-linear distortion is derived from the amplifier 1.
FIG. 3 shows the distortion compensating circuit of the predistorter method of the conventional technology. In this method, an input signal is supplied to a distortion forming circuit 6, and a distortion signal for canceling the non-linear distortion generated in the power amplifier 1 is produced. This distortion signal is added to the originally supplied input signal in an adding circuit 7, and thereafter is supplied to the power amplifier 1.
FIG. 4 shows the distortion compensating circuit of the feed-forward method of the conventional technology. In this method, the differential component between the input signal of the amplifier 1 and the output signal thereof, i.e. the non-linear distortion component, is detected by a subtraction circuit 5. This distortion component is amplified to the level of the non-linear distortion component contained in the output signal of the amplifier 1 by an amplifier 8, and is then supplied to a subtraction circuit 9, and the non-linear distortion component contained in the output signal of the amplifier 1 is removed.
With the negative feedback method, even when the parameters of the amplifier 1 change due to temperature characteristics or age deterioration, etc., the distortion compensating characteristic hardly deteriorates at all; however, because the negative feedback method is a closed loop control method, when the linearity of the loop is not maintained, its operation can become unstable and oscillation can occur. Then, when the power source voltage and thereby the gain of the amplifier 1 is controlled based upon the distortion detection output from the subtraction circuit 5, the change in the gain of the amplifier 1 with respect to this detection output does not become linear. Therefore, with the negative feedback method, in order to avoid such oscillation and maintain stable operation, the loop gain must be kept low. As a result, the distortion compensating amount cannot be made large.
As to this point, with the predistorter method and the feed-forward method, because both of these methods involve open-loop control, there is no stability problem. However, if the parameters of the amplifier 1 change due to the temperature characteristics or age deterioration, the distortion compensating characteristic deteriorates. Furthermore, with the feed-forward method, the distortion component must be faithfully amplified by the amplifier 8, and linearity of the amplifier 8 is required.
The present invention aims to solve these problems.
As previously described, the non-linear distortion which occurs in the power amplifier 1 is caused by non-linearity of the input/output characteristic of the power amplifier 1. From a different point of view, it may be conceived that such non-linear distortion is caused by gain error of the power amplifier 1. Then, if it is gain error that causes the non-linear distortion, by detecting the gain error and correcting the level of the input signal based upon the detected gain error, the gain error can be canceled.
This invention uses this approach to compensate for the non-linear distortion of a power amplifier.
Specifically, in the present invention, in a distortion compensating circuit of a high-frequency power amplifier 13 for compensating the non-linear distortion of the high-frequency power amplifier 13 by negative feedback, there are provided (the reference numerals correspond to components of the preferred embodiments discussed hereinafter): a variable gain circuit 12, provided at a front stage of the high-frequency power amplifier, for variable-controlling the level of an input signal S12 to be supplied to the high-frequency power amplifier 13; a gain detecting circuit 25 for detecting the gain of the high-frequency power amplifier 13; and a loop filter 28 for supplying the output signal S24 of this gain detecting circuit 25 to the variable gain circuit 12 as a control voltage thereof. The gain detecting circuit 25 is made up of: a first logarithm converting circuit 22 for logarithmically converting an envelope detection signal S21 of a signal S11 inputted into the variable gain circuit 12; a second logarithm converting circuit 23 for logarithmically converting an envelope detection signal S27 of an output signal S13 from the high-frequency power amplifier; and a subtraction circuit 24 for outputting a subtraction signal S24 of the respective output signals S22 and S23 of the first and second logarithm converting circuits 22 and 23. The control voltage vs. gain characteristic of the variable gain circuit 12 has an inverse logarithm characteristic.
The non-linear distortion of the high-frequency power amplifier 13 is detected, and this detection signal S24 is negative-fed back to the variable-gain circuit 12, and the non-linear distortion is compensated.